The present invention relates generally to amplification of optical signals and, more particularly, to methods and devices for minimizing the polarization dependent gain in optical amplifiers by amplifying optical signals using a depolarizer in conjunction with a semiconductor optical amplifier (SOA).
Technologies associated with the communication of information have evolved rapidly over the last several decades. Optical information communication technologies have evolved as the technology of choice for backbone information communication systems due to, among other things, their ability to provide large bandwidth, fast transmission speeds and high channel quality. Semiconductor lasers and optical amplifiers are used in many aspects of optical communication systems, for example to generate optical carriers in optical transceivers and to generate optically amplified signals in optical transmission systems. Among other things, optical amplifiers are used to compensate for the attenuation of optical data signals transmitted over long distances.
There are several different types of optical amplifiers being used in today's optical communication systems. In erbium-doped fiber amplifiers (EDFAs) and Raman amplifiers, the optical fiber itself acts as a gain medium that transfers energy from pump lasers to the optical data signal traveling therethrough. In semiconductor optical amplifiers (SOAs), an electrical current is used to pump the active region of a semiconductor device. The optical signal is input to the SOA from the optical fiber where it experiences gain due to stimulated emission as it passes through the active region of the SOA.
Like other devices employed in optical networks, SOAs suffer from polarization sensitivity. That is, the gain experienced by a light beam that is input to a conventional SOA will vary depending upon the polarization state of the input optical energy. In this context, the polarization state of a light beam is typically described by the orthogonal polarization components referred to as transverse electric (TE) and transverse magnetic (TM). Unfortunately, even if light having a known (e.g., linear) polarization state is injected into a typical optical fiber (i.e., a single mode fiber), after propagation through the optical fiber the light will become elliptically polarized. This means that the light input to SOAs placed along the optical fiber will have TE and TM polarization components of unknown magnitude and phase, resulting in the gain applied by SOAs also varying indeterminately as a function of the polarization state of the input light.
There are various techniques that have been employed to compensate for the polarization dependent gain that is introduced by SOAs. One such technique, shown in FIG. 1, is to arrange two SOAs in series. In amplifier 10, the gain for TE mode light is greater than the gain for TM mode light. Amplifier 12 has the same structure as amplifier 10 but is rotated by 90 degrees so that the gain for TM mode light is greater than the gain for TE mode light, i.e., in reverse proportion to the polarization gain ratio for amplifier 10. In this way, the optical energy output from the combination of amplifiers 10 and 12 is substantially polarization independent. This technique can also be practiced by arranging the SOAs in parallel as described, for example, in the textbook Optical Amplifiers and their Applications, edited by S.Shimada and H. Ishio, published by John Wiley & Sons, Chapter 4, pp. 70–72, the disclosure of which is incorporated here by reference. Another technique for compensating for polarization dependent gain is to use some other corrective device downstream of the SOA as shown in FIG. 2. For example, a variable polarization dependent loss control device 22 can be disposed downstream of the SOA 20 to compensate for unequal magnitudes of TE and TM gain. This technique is described in U.S. Pat. No. 6,310,720, the disclosure of which is incorporated here by reference. Both of these techniques suffer from, among other things, the drawback of requiring a number of additional components to create a single polarization insensitive SOA, thereby increasing the cost of the solution.
Attempts have also been made to provide an integrated solution to this problem, i.e., to design polarization insensitive SOAs. One such attempt is described in U.S. Pat. No. 5,982,531 to Emery et al., the disclosure of which is incorporated here by reference. Therein, the active material in the SOA is subjected to a tensile strain sufficient to render the amplifier insensitive to the polarization of the light to be amplified. However, balancing the TE/TM gain using such techniques requires extremely accurate control over device geometry, layer thickness, layer composition and background absorption loss. In practice, this level of control is very difficult to achieve in a repeatable manufacturing process, i.e., there may be a significant variance in the polarization sensitivity of SOAs manufactured using such techniques from one manufacturing run to another.
Accordingly, Applicants would like to provide methods and devices that amplify optical signals in a manner which is relatively polarization insensitive, but which also facilitates manufacturing repeatability for amplification devices and, therefore, is cost effective.